(1) Field of the Invention
The present invention relates to a DC--DC converter, and more particularly to a DC--DC converter provided with a so called AC link, in which the controlled DC current is first converted into an AC current by using an inverter and which AC current is then converted back into a DC current again.
(2) Description of the Prior Art
In the conventional DC--DC converters, various types have been used in practice. In one of such known converters, there is known a chopper type DC--DC converter incorporating a chopper circuit, which can produce a DC voltage higher than or lower than the input DC voltage. The former is known as the step-up chopper converter and the latter is known as the step-down chopper converter.
As a practical embodiment, FIG. 1a shows diagrammatically such a step-up type converter and FIG. 1b shows such a step-down type converter.
In FIGS. 1a and 1b, 1 and 1' designate controlled rectifiers, which operate to short or open responsive to an outer control signal. Example of these elements are transistors, gate turn-off (GTO) thyristors, and thyristors provided with a forced turn-off circuit. 2 and 2' are rectifiers, 3 and 3' are DC smoothing reactors, 4 and 4' are DC smoothing capacitors, P.sub.1 and N.sub.1 are the input terminals and P.sub.2, N.sub.2, P.sub.2 ' and N.sub.2 ' are the output terminals.
These types of converters are known to have the following advantages and also disadvantages.
(i) Step-up type chopper converter
This converter is effective only for the step-up of the voltage.
(a) In a use such as an electric motor vehicle using high input DC voltage, the flexibility is very small since the DC output voltage becomes a still higher DC voltage. PA1 (b) Ripple current component is larger since the current fed to the smoothing capacitor flows intermittently. PA1 (c) The ripple in the input current can be made comparatively small. PA1 (a) In case of the electric vehicle having a wide fluctuation range of the input voltage, the flexibility or applicability is small since the output voltage must be selected less than the minimum value of the input voltage. PA1 (b) The current flowing through the smoothing reactor 3' is comparatively larger compared with the step-up type so that the weight and size of the smoothing reactor portion is larger than the other type. PA1 (c) The ripple current is large since the input current passed by the chopper is intermittent and this affect for the input current supply source. PA1 (i) By the step-up and step-down action of the transformer 5 provided intermediary to couple both circuits, the output voltage can freely be chosen. PA1 (ii) Compared with an older system in which the output DC voltage is controlled, the DC output voltage obtained by rectification of the AC output produced by the pulse width modulation flows intermittently so that the smoothing reactor 31 and the smoothing capacitor 41 are inevitable. This affects the input source so that the input current flows also intermittently and thus the ripple current is large. PA1 (iii) The capacity of the transformer 5 should match the capacity of the output load.
(ii) Step-down type chopper converter
This converter is effective only for the step-down of the voltage. By this reason it has following features.
Furthermore, one example of a conventional DC--DC converter having inverter portions and provided with an AC link intermediary in which AC current flows, is shown in FIG. 2 and this embodiment will be explained hereinafter.
FIG. 2 shows such a conventional DC--DC converter provided with an AC link. In FIG. 2, elements 11, 12, 13 and 14 are controlled rectifiers having the same function as the elements 1 and 1' shown in FIGS. 1a and 1b. Then, elements 21, 22, 23, 24, 25, 26, 27 and 28 are rectifiers. Elements 31 and 41 are the smoothing reactor and the smoothing capacitor. The circuit is divided into 2 parts which are coupled by a transformer 5. P.sub.11 and N.sub.11 designate DC input terminals, AC.sub.1, AC.sub.2, AC.sub.3 and AC.sub.4 are AC terminals and P.sub.21 and N.sub.21 designate DC output terminals.
This device itself is a well known one so that a detailed explanation is omitted. But the operation is explained just briefly.
From a circuit shown left side and including the controlled rectifiers 11 to 14 and the rectifiers 21 to 24, a voltage type single phase bridge inverter is formed, which inverter controls the AC output voltage by pulse width modulation when the voltage is to be controlled. This AC output voltage is transformed by the transformer 5 and converted into a DC voltage by a rectifier circuit formed from rectifiers 25 to 28. Thereafter this output current is smoothed by the smoothing reactor 31 and the smoothing capacitor 41 and then fed to output P.sub.21 and N.sub.21 as a DC voltage.
Such a conventional DC--DC converter provided with an AC link has the following advantages and disadvantages.